1. Field of the Invention
The present invention relates to an automatic focusing device for a camera, for moving a lens to a in-focus position by driving a motor, in response to an output signal of a focus detecting device indicating the defocus amount the in-focus position.
2. Related Background Art
There is already proposed, for example in the Japanese Patent Laid-open No. (94334/1981) corresponding to the German Patent Offenlegung No. 3049233 or the British Pat. No. 2069286, and in the Japanese Patent Laid-open No. (18611/1984) corresponding to the U.S. Pat. No. 4,451,729, an automatic focusing device capable of precisely moving a photographing lens to the in-focus position within a short time, by detecting, while a driving motor is stopped, the required amount of movement for driving the photographing lens to the in-focus position, corresponding to the amount of defocus of said lens, and driving said motor at a high speed for moving the photographing lens to the focused state if said required amount of movement is large, indicating that the lens is positioned far from the in-focus position, or driving said motor at a low speed if said lens has come close to the in-focus position or said lens is originally placed within a predetermined range close to the in-focus position so that said required amount of movement is small. Also the present applicant proposed, in the U.S. patent application Ser. No. 688,355 of Jan. 2, 1985, an automatic focusing motor control method utilizing servo drive. Said servo control method provides a higher driving speed when the amount of defocus is large and a lower driving speed when said amount of defocus becomes smaller.
At first reference is made to FIG. 4 for explaining the process of stopping the photographing lens at the in-focus position, in these known devices.
In FIG. 4, the ordinate indicates the driving speed of the motor while the abscissa indicates said required amount of movement, and a point P.sub.0 corresponds to the focused state.
Because of its characteristics, the motor reaches a stable high-speed drive state only after a predetermined amount of movement (corresponding to A in FIG. 4) from the start of drive, and, it cannot be stopped immediately in response to a stop signal but only after a predetermined amount of movement (corresponding to B in FIG. 4).
Therefore, when the lens is at a position P.sub.1 corresponding to a large amount of defocus and therefore requiring a large amount of movement, the motor is at first driven at a high speed N.sub.H, and is decelerated from a position P.sub.L which is located in front of the in-focus position P.sub.0 by a predetermined amount. The motor then continues to be driven at a low speed N.sub.L to a position P.sub.S which is located in front of said in-focus position by a predetermined distance (corresponding to C) which the lens has to travel before stopping when it is braked at the low driving speed N.sub.L, and the motor is braked when the lens reaches said position P.sub.S whereby it is stopped at the in-focus position P.sub.0.
On the other hand, when the lens is located at a position P.sub.2 which is closer than the position P.sub.L to the in-focus position P.sub.0, corresponding to a small amount of defocus and thus requiring a small amount of movement, the motor is driven at the low speed N.sub.L from the beginning.
However, if the lens is located at a position P.sub.3 of a medium amount of movement somewhat farther than the position P.sub.L, the motor is accelerated according to the start-up characteristics thereof but the lens reaches the position P.sub.L before the high driving speed N.sub.H, and the motor is switched to the deceleration process. There is therefore a very large change in the acceleration, causing a large shock and giving an unpleasant feeling to the user.